1. Field of the Invention
The present invention relates to an optical wavelength filter to be used for various devices for optical communications, in particular, to an optical wavelength filter of band rejection type in which a plurality of optical filter sections each blocking a light of required wavelength from passing through, are cascade connected to be in a multi-staged structure.
2. Related Art
There have been demanded developments of optical communication systems and optical signal processing systems capable of constructing networks of large capacities and ultra-long distance with an explosive increase of IP data communication demand. In a transmission system adopting a wavelength-division multiplexing (WDM) transmission as a basic technique thereof, it is possible to realize the large capacity transmission and easily perform the division-multiplication with the wavelength as a unit, so that the construction of flexible optical networks that performs multiplication-division of different kinds of services at optical levels, such as, optical cross-connecting (OXC), optical add/drop multiplexing (OADM) and the like. Therefore, the development and manufacturing of transmission apparatus and signal processing apparatus using the above system have been remarkably made.
In these apparatuses, there are used many optical function devices, such as, an optical wavelength filter that separates a signal light for each wavelength, and the like. Specifically, the optical wavelength filter is used, for example, for the wavelength switching in the OXC and OADM, the separation of each wavelength at a receiving section, the ASE cutting, and the like.
By forming the above mentioned optical wavelength filter on a substrate made of SiO2, LiNbO3 and the like, it becomes possible to achieve the high functions, down-sizing, integration, reduction of electricity, and reduction of cost. Further, in a case where a plurality of optical wavelength filters integrated on a substrate are cascade connected, it is possible to achieve the narrow transmission band and the improvement of suppression ratio between other channels during used as band-pass type optical wavelength filters, and also it becomes possible to achieve the improvement of extinction ratio during used as band-rejection type optical wavelength filters (notch filters).
For the wavelength characteristics of the rejection type optical wavelength filter to be used for optical apparatuses, such as OXC, OADM and the like, as mentioned above, for example, as shown in the conceptual diagram of FIG. 18A, it is an ideal to have a filter characteristic that is changed in rectangular, namely, a change in transmissivity from the passing band to the blocking band is steep and also the blocking band has a required width. The multi-staged structure in which the above plurality of optical wavelength filters are cascade connected is considered to be useful as means for achieving the realization of the above described ideal filter characteristics. For example, if acousto-optic tunable filters are used as the optical wavelength filters at respective stages, it is known that, basically, the filter characteristic having an excellent extinction ratio can be obtained, as the number of stages of AOTFs is increased.
However, in a case where the multi-staged structure as described above is adopted, if the wavelengths (selected wavelengths) of lights to be blocked from passing through at the optical wavelength filters at respective stages are all coincident, as shown in the conceptual diagram of FIG. 18B, since the transmissivity becomes minimum at one point, the width of blocking band becomes narrower. For the blocking band of the rejection type optical wavelength filter, a required width needs to be ensured, considering the conditions of, for example, the wavelength width of optical signal corresponding to the spectrum width of light source such as laser, errors in setting or controlling of optical wavelengths filters, or the unstable wavelength of light source. Therefore, according to the filter characteristics as shown in FIG. 18B, it becomes impossible to block the passing of optical signal of desired wavelength even in a case a slight variation is caused in the setting of the optical signal wavelength or the setting of filter.
The present invention has been achieved in view of the above problems, and an object of the present invention is to provide an optical wavelength filter of band rejection type capable of reliably and stably blocking a light of required wavelength from passing through.
In order to achieve the above object, the present invention provides an optical wavelength filter comprising a plurality of optical filter sections each blocking a light corresponding to a selected wavelength from passing through, the plurality of optical filter sections being cascade connected to be in a multi-staged structure, wherein respective selected wavelengths of the plurality of optical filter sections are deviated from one another, to form a blocking band having the wavelength band corresponding to a deviation amount between the selected wavelengths.
According to the optical wavelength filter as mentioned above, a characteristic in which transmission wavelength characteristics in the optical filter sections at respective stages are overlapped with one another can be obtained in the entire filter, and there is formed a blocking band having the wavelength band corresponding to the deviation amount of selected wavelengths of the respective optical filter sections. Thus, even if the wavelengths of optical signal, the filter setting and the like fluctuate, it is possible to reliably and stably block an optical signal of desired wavelength from passing through. Further, since the plurality of optical filter sections are in the multi-staged structure, it is possible to obtain a filter characteristic having an excellent extinction ratio.
In the above optical wavelength filter, each of the optical filter sections may include an acousto-optic filter that blocks a light corresponding to a selected wavelength from passing through by utilizing the acousto-optic effect. Moreover, as specific means for deviating the selected wavelengths in the respective acousto-optic filters from one another, it is possible to adopt means to differ widths of optical waveguides constituting the respective acousto-optic filters from one another, to give RF signals of different frequencies to the respective acousto-optic filters, to change propagation directions of acousto-waves to propagation directions of lights in the respective acousto-optic filters, or to differ acoustic velocities of respective acousto-wave guides constituting the respective acousto-optic filters.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.